1. Field of the Invention
The present invention relates, generally, to laser pointing systems, more especially to wide angle laser windows for pointing a coherent radiation beam, and most particularly to such a window for directing a laser beam through the skin of an aircraft whether in a transmission or reception mode.
2. Description of the Background Art
Laser devices find wide ranging utility on board an aircraft. The same might be employed as a primary weapon system, where a relatively high power device is required to transmit its coherent radiation beam to a target destined for destruction. Lasers are adapted for ranging systems, requiring the transmission of the radiation beam to a specific target and then the reception and processing of a reflection therefrom. Yet another mode of operation includes tracking or homing from a distant point, where reception within the aircraft of a beacon signal is of paramount interest. Regardless of the specific intended utility for the laser system, the coherent radiation beam must pass through the skin of the craft; outwardly, inwardly, or both depending upon the objective. And too, the azimuthal position of either the target or the source of radiation (depending upon the mode of operation) typically may vary during the course of flight; making very desirable the ability to maintain a steady course irrespective of those variations as opposed to the need for course changes to present the distant point at a convenient location vis-a-vis the craft.
The problem of propagating a laser beam inwardly or outwardly from an aircraft is easy to articulate but elusive to solve. Heretofore a proposal to achieve this aim was the inclusion of a turret projecting outwardly of the aircraft and rotatably displaceable with respect thereto. Placing the laser optics within such a turret permits beam pointing to a distant location. However, the associated turbulence created by the projection of the turret into the airstream aft of its location on the craft has virtually eliminated the ability to transmit or receive a laser beam in a rearward aspect with respect thereto. This has led to the need for development of an efficient alternative to a turret or other device which projects into the airstream and thereby presents a non-aerodynamic surface.
The solution proposed by the present invention is one which adapts to good utility in the context aforesaid the basic concept of a liquid prism. Liquid prisms, per se, are known in the prior art for various purposes. In capsule sum, however, none of the known devices is configured for steering a laser beam, let alone one which must pass through the skin of an aircraft.
U.S. Pat. No. 3,337,287 generally typifies one type of fluid lens system of the prior art. The device disclosed in that patent is associated with a projection system for films where the image of a motion picture must be fixed on a screen. One of the principal objectives is to minimize deflection of the projected image arising, inter alia, from wear or change in the film, vibration or external forces, or the like. The proposal to resolution of that problem is the use of a variable angle prism in the form of a fluid-filled cell having a pair of aligned, pivotally mounted windows responsive to movement through a servo system. Distention of the prism components permits for small, angular variation in the transmission path of projected light, thereby stabilizing the incoherent image beam projected to a screen.
Another type of fluid lens is disclosed in U.S. Pat. No. 3,161,718 where an objective, in part, is to replicate the manner in which the human eye focuses. Numerous variable power fluid lens configurations are suggested, where the lens elements themselves include planar, convex and concave geometries or various combinations thereof. A liquid, or even a suitable gas, having proper optical characteristics, including glycerine and gelatin, is charged to a fluid chamber in the optical path of the lens system. A tensioning device controls a diaphragm in order to achieve desirable focusing effects. Conceptually similar to the '718 patent is U.S. Pat. No. 2,836,101 which likewise provides means for producing a variable focal length lens for incoherent light. Neither of these devices, nor those similar thereto, strictly involves beam steering of any sort, let alone the steering of a coherent beam of radiation.
A fiber optics scanning system is disclosed in U.S. Pat. No. 3,389,950. The device includes a type of spherical lens system intermediate closely mated optical surfaces. A liquid is employed to reduce the reflection between the two optical surfaces. Although the system includes what might be deemed a variable angle prism, it essentially is in the nature of a solid prism; the presence of a fluid to the contrary notwithstanding.
U.S. Pat. No. 4,289,379 discloses another variety of a variable focal length lens including a fluid medium within the optical path. It is noteworthy insofar as it includes a piezoelectric element which modifies the curvature of the lens in response to an input voltage. For still another type of variable focal length lens system, attention is invited to U.S. Pat. No. 3,641,354.
Quite a number of other patented devices of the foregoing ilk appear in the art. While each undoubtedly provides its own respective advantage within its intended field of use, none suggests or discloses a beam steering apparatus for laser pointing. In further point of fact, none discloses or suggests a beam steering system capable of replacing those associated with aircraft in a manner overcoming the undesirable non-aerodymanic projection of, e.g., a turret. Accordingly, the need exists to adapt the general concept of fluid lenses to a specific structural implementation, converting the same to a fluid prismatic window which will efficiently and reliably achieve the goal of a wide angle airborne laser window.